Currently, liquid crystal displays are regarded as the most competitive display type, capable of replacing the Braun tube by virtue of its advantages of light weight and low power consumption. Particularly, in a thin-film transistor liquid crystal display (TFT-LCD) actuated using a thin-film transistor, the independently actuated pixels result in excellent response speed of the liquid crystal, which makes it possible to obtain high-definition moving picture; thus, the scope of its application has gradually expanded into notebook computers, wall-mounted TVs, and the like.
In production of a typical color thin film transistor-liquid crystal display, a thin-film transistor actuating device and an ITO transparent electrode are laminated on a glass substrate and an alignment film is subsequently layered thereon to form a lower plate of a cell. Spacers are formed on internal surfaces of a pair of upper and lower substrates using a sealant to inject the liquid crystal material therebetween. Polarized films are attached to external surfaces of the glass substrates. The liquid crystal material is injected and hardened between a pair of substrates to produce a liquid crystal display cell.
In the TFT-LCD, in order to use the liquid crystal as an optical switch, the liquid crystal must be initially aligned in a predetermined direction on a layer, in which the thin film tansistor is formed at an innermost portion of the display cell. For this, a liquid crystal alignment film is used.
A method of producing the alignment film is exemplified by a rubbing treatment method, in which a polymer resin film, such as a polyimide resin, formed on a substrate, is unidirectional rubbed using cloth and the like, or a method in which silicon dioxide (SiO2) is inclinedly deposited. However, the alignment film produced using the rubbing treatment method has problems of contamination by impurities, reduction in yield of products due to static electricity, and degradation of contrast because of the contact during the rubbing. Additionally, the method using the inclined deposition is problematic in that production cost is high and it is unsuitable to apply to a large liquid crystal display because it is difficult to assure a large area.
In order to avoid these problems, an alignment method has been developed. The method comprises a non-rubbing process using an alignment material for photopolymerization in which light is radiated to cause photopolymerization so as to induce alignment of a polymer and thereby aligning the liquid crystal. A representative example of the non-rubbing processing is photoalignment caused by photopolymerization, which was announced by M. Schadt et al. (Jpn. J. Appl. Phys., Vol 31, 1992, 2155), Dae S. Kang, et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000). The photoalignment has a mechanism in which photosensitive groups combined with the polymer, using linearly polarized ultraviolet rays, cause a photoreaction, and in this procedure a main chain of a polymer is aligned in a predetermined direction, and thereby aligning the liquid crystal.
Polycinnamate-based polymers, such as PVCN (poly(vinyl cinnamate)) and PVMC (poly(vinyl methoxycinnamate)), have been mainly used as a material of the alignment film for photopolymerization. However, these polymers are problematic in that thermal stability is poor even if photoalignment is excellent. In other words, thermal stability of the alignment film depends on thermal stability of the polymer, and thermal stability of the alignment film is poor because the poly(vinyl cinnamate)-based polymer generally has a glass transition temperature of 100° C. or less at a main chain thereof.
Meanwhile, Japanese Patent Laid-Open Publication No. Hei. 11-181127 discloses a method of producing a polymer-type alignment film, which includes a main chain, such as acrylate and methacrylate, and a side chain having a photosensitive group, such as a cinnamic acid group, and an alignment film produced using the method. However, it is disadvantageous in that since the polymer as disclosed in the above document has poor mobility, it is difficult to obtain desired alignment characteristics even though it is exposed to light for a long time. The reason for this is that the photosensitive group existing in the polymer is restricted by the main chain of the polymer, thus it is difficult for it to rapidly react to polarized light. Hence, as it takes a long time to form a network polymer, the processing efficiency is reduced. Additionally, if the alignment is conducted for an insufficient length of time, alignment of the liquid crystal of the resulting liquid crystal display is poor, thus, undesirably, dichroic ratio is small and contrast is degraded.